Atomically dispersed MoNi alloy catalyst for partial oxidation of methane

The catalytic partial oxidation of methane (POM) presents a promising technology for synthesizing syngas. However, it faces severe over-oxidation over catalyst surface. Attempts to modify metal surfaces by incorporating a secondary metal towards C–H bond activation of CH4 with moderate O* adsorption have remained the subject of intense research yet challenging. Herein, we report that high catalytic performance for POM can be achieved by the regulation of O* occupation in the atomically dispersed (AD) MoNi alloy, with over 95% CH4 conversion and 97% syngas selectivity at 800 °C. The combination of ex-situ/in-situ characterizations, kinetic analysis and DFT (density functional theory) calculations reveal that Mo-Ni dual sites in AD MoNi alloy afford the declined O2 poisoning on Ni sites with rarely weaken CH4 activation for partial oxidation pathway following the combustion reforming reaction (CRR) mechanism. These results underscore the effectiveness of CH4 turnovers by the design of atomically dispersed alloys with tunable O* adsorption.


Figure S2 |
Figure S2 | Synthetic methods for alloy.Scheme of atomically dispersed MoNi alloy synthesis by wet-chemistry method under N2 flow.

Figure S4 |
Figure S4 | Phase diagram for Mo-Ni alloys.Phase diagram of Ni-rich Mo-Ni alloys (70-90 at.% Ni), and x axis refer to atomic content and y axis stands for temperature.Adapted from R. E. W. Casselton and W. Hume-Rothery, Journal of the Less-Common Metals.1964, 7, 212-221.

Figure S5 |
Figure S5 | Identification of valance state for fresh and spent catalysts.(a) XPS spectra of Ni 2p showing valence state variation for Ni/SBA-15 and AD MoNi alloy/SBA-15.Mo 3d spectra for fresh (b) and spent (c) AD MoNi alloy/SBA-15.

Figure S6 |
Figure S6 | XAFS analysis and fitting for electronic and coordination structure.(a) XANES spectra, (b) FT of k 3 -weighted EXAFS R-space spectra and (c) k space for MoNi alloy and NiO reference.(d) XANES spectra, (e) FT of k 3 -weighted EXAFS Rspace spectra and (f) k space for fresh and spent AD MoNi alloy and Ni foil.FT of EXAFS R-space fitting for (g) Ni foil, (h) fresh AD MoNi alloy and (i) used AD MoNi alloy.

Figure S7 |
Figure S7 | XAFS fitting and treatment.EXAFS R-space fitting for (a) Ni foil, (b) AD MoNi alloy and (c) 5Ni1Mo alloy.Fitting window from 1.0 to 3.0 Å marked with green line.Wavelet transform (WT) of (d) Ni foil, (e) AD MoNi alloy and (f) 5Ni1Mo alloy, indicative of single peak feature.

Figure S9 |
Figure S9 | Performance comparison between AD Mo/Ni alloy/SBA15 and Ni/SBA15 On-stream profiles of (a) Conversion and (b) Selectivity, and comparison of (c) Conversion and (d) Selectivity with GHSV of 15,000 mLCH4 gcat −1 h −1 from 800 to 700 °C for AD Mo/Ni alloy/SBA15 and Ni/SBA15, respectively.On-stream profiles of (e) Conversion and (f) Selectivity, and comparison of (g) Conversion and (H) Selectivity with GHSV of 24,000 mLCH4 gcat −1 h −1 from 800 to 700 °C for AD Mo/Ni alloy/SBA15 and Ni/SBA15, respectively.

Figure S11 |
Figure S11 | Elemental distribution for fresh and spent catalyst.EDS mapping for (a) Ni K, (b) O K, (c) merge of pure Ni/SBA-15 catalysts (d) XRD for deactivated, spent (20h test) and fresh Ni/SBA-15 catalysts, and the deactivated, spent (20h test) and fresh AD MoNi alloy/SBA-15 catalysts.EDS mapping of (e) Ni K, (f) Mo L, (g) O K and (h) merge of AD MoNi alloy/SBA-15.EDS mapping of (i) Ni K, (j) Mo L, (k) O K and (l) merge of spent AD MoNi alloy/SBA-15 for POM reaction.

Figure S13 |
Figure S13 | Control experiments for identification of POM mechanism.Long-term test for (a) DRM and (b) DRM+SRM.(c) Temperature distribution located in gas entry bed (c) and gas exit bed (d) with obvious temperature drop.Scale bar is given within the digital image.

Figure S15 |
Figure S15 | Deactivation rate correction and kinetics tests.Runlog image for (a) AD MoNi alloy/SBA-15 and (b) Ni/SBA-15 to calculate the reaction order of O2.Indicative of Ni/SBA-15 gradual deactivation and AD MoNi alloy/SBA-15 rarely deactivated when conducting kinetics study at 800 °C.The term ri (i=1,2,3) in (a) is the real forward rate without correction due to the rarely deactivated behavior.The term r'i,0 (i=1,2,3,4) in (b) is the corrected forward rate without deactivation at zero time.(c) Reaction order of O2 for pure Ni/SBA-15 after correcting rate could be observed in Figure 4D.(d) Runlog for Ni/SBA-15 where red dots represents corrected forward rate without deactivation at zero time (e) Reaction order of O2 at 670 °C (f) Arrhenius plot for AD MoNi alloy/SBA-15 and Ni/SBA-15 from 790 to 770 °C.

Figure S16 |
Figure S16 | Formation energy for possible structure determination.(a) Energies of surface models with 2 Mo atoms at different position.Inset represent corresponding schematic diagram (The structure marked by purple circle represents the most stable structure).Possible structure for AD MoNi alloy and its energy, where the two Mo atoms located at sub-surface for MoNi structure was the most stable structure with most negative energy.(b) Energy difference of -0.72 eV between 1Mosub and 1Mosurf with a single Mo atom.

Figure S17 |
Figure S17 | Segregation energy for possible structure determination.Segregation energy of MoNi (100) with different amount of adsorbed O species.

Figure S18 |
Figure S18 | Gibbs free adsorption energy.The variations of average oxygen Gibbs free adsorption energy on 1Mosurf-1Mosub surface with different surface oxygen coverage.

Figure S19 |
Figure S19 | Phase diagram for Ni.(a) Phase diagram for possible structure with practical O2 partial pressure of ~0.125 under reaction condition at 800 °C with partial pressure of CH4:O2:N2=2:1:5.(b) Possible structure according to phase diagram.

Figure S20 |
Figure S20 | Ab initio molecular dynamics (AIMD) for AD MoNi alloy.(a-c) Three repeated AIMD simulations were conducted at 1073 K within the NVT ensemble for 20ps.(d-f) The atomic structures represent MoNi (100) after simulation.Pre-adsorbed O atoms remain stable for 20 ps under reaction condition.

Figure S23 |
Figure S23 | Reaction pathways for MoNi (100) and Ni (100).Reaction pathways of C* with O* during methane reforming on (a) MoNi and (b) Ni.(c) The dissociative adsorption energy of O2 on pure MoNi, pure Ni and MoNi-2O.

Table S1
EXAFS fitting parameters at the Ni K-edgeNote: a CN: coordination numbers; b R: bond distance; c σ 2 : Debye-Waller factors; d R factor: goodness of fit.e ΔE0: the inner potential correction.Ѕ0 2 was set as 0.98, which was obtained from the experimental EXAFS fit of NiO by fixing CN as the known crystallographic value and was fixed to all the samples.Table S2 EXAFS fitting parameters at the Ni K-edge for reacted sampleNote: a CN: coordination numbers; b R: bond distance; c σ 2 : Debye-Waller factors; d R factor: goodness of fit.e ΔE0: the inner potential correction.Ѕ0 2 was set as 0.98, which was obtained from the experimental EXAFS fit of NiO by fixing CN as the known crystallographic value and was fixed to all the samples.
c R factor d ΔE0 (eV) e c R factor d ΔE0 (eV) e